In the prior art temperature controlled device of a heating blanket, the difference of temperature must be controlled within a predetermined range to avoid overpower conditions so as to assure a safety operation. However current commercial heating blanks are not so good at this aspect so that they are possible to induce fire accidents or to burn the users. Thereby there is an eager demand for a novel design which can improve such kinds of defects.
Referring to FIG. 6, it is illustrated that in the prior art, the conduction time period is controlled by conduction of a part of a cycle, for example, in FIG. 6, the power is conducted from 90 degrees to 180 degrees in each cycle. It is shown that only one fourth of one cycle is conductive. Thereby the power ratio is 25% of the whole power. However the signal is conducted from some part of the wave instead of from the zero points of the wave. As a result, some harmonic waves are generated and thus the electromagnetic radiation is high. In some areas, the radiation of the heating blanket is limited by laws. Thereby the radiation due to the harmonic waves is often too great to be accepted.